Correct Answer: speed of rotation

Solution : The correct answer is  tachometer

A tachometer is an instrument designed to measure the rotation speed of a shaft or disk. Tachometers generally measure rotations per minute (RPM) though some models also serve as rate meters and/or totalizers.

tachometer, device for indicating the angular (rotary) speed of a rotating shaft. The term is usually restricted to mechanical or electrical instruments that indicate instantaneous values of speed in revolutions per minute, rather than devices that count the number of revolutions in a measured time interval and indicate only average values for the interval.

Mechanical tachometers utilize the fact that the centrifugal on a rotating mass depends on the speed of rotation and can be used to stretch or compress a mechanical spring.

Final answer is  tachometer